The glass sheet which is molded and transferred in its ribbon shape is kept in a keeping container such as a palette or shipped in a state where it is cut in a square shape and a desired size. In this case, according to a using purpose, an end face edge of the glass sheet after cut is chamfered (seaming) using e.g. a diamond wheel, or otherwise the end face is polished so that the end face becomes an arc-shape in section thereby providing a semi-transparent polished glass. As a result, occurrence of injury in handling the glass sheet is prevented, thereby assuring safety. Further, damaging or reduction in a product value due to reduction in the end face of the glass sheet can be prevented.
Where such seaming processing or end-face processing is executed, as the case may be, at the edge of the glass sheet, the scratch or chip may be generated owing to poor polishing or blocking resulting from abrasion of a diamond wheel.
Particularly, in a process for manufacturing a glass sheet used as a display substrate for a flat display panel for e.g. a liquid crystal display, plasma display, field-emission display, or organic EL display, if there is a fine scratch or chip in the seaming area of the end face of the glass sheet when the glass sheet is heat-treated at a high temperature, a trouble of cracking of the glass sheet may occur.
There are various disclosed techniques for detecting the defect on the end face of the glass sheet. For example, JP-A-2001-153816 discloses a method for detecting an edge defect on a glass sheet in which with the edge of the glass sheet polished after cut is being placed between a light source and a camera, incident light is applied to the edge from the light source to light up the defect at the edge, and the defect is image picked up by the camera, the image thus acquired is processed to investigate the presence/absence of the defect. The light source is arranged behind the glass sheet when seen from the camera so that it is outside the visual field of the camera. The quantity of light which reaches the camera from the light source is limited so that the surface of the glass sheet and the common face of the edge in the image is dark and the chip residing at the edge is bright, thereby optically detecting the chip.
Further, JP-A-06-258231 filed by the applicant of this application discloses a device for detecting the defect at an edge of a glass sheet having a seaming face chamfered at the edge in a state where it is placed horizontally. This detecting device includes a light source for emitting light to the edge from two upper and lower diagonal directions on the side opposite to the glass sheet, and at least two cameras for picking up the edge at the corner on the light source side of the glass sheet face and seaming face, through a transparent area of the glass sheet, from outside the extended area of an optical path of the light applied to the edge and the side opposite to the light emitting direction. The defects at the edge are discriminated by the magnitude of the bright signal acquired from image-pick-up by the cameras.
Further, JP-A-2003-247953 discloses a method and device for checking the external appearance of a liquid crystal panel. In the device for checking the external appearance of the liquid crystal panel, a ring illuminating lamp having a diameter larger than the external size of the liquid crystal panel is arranged on the outer periphery of the liquid crystal panel. The method for checking the external appearance of the liquid crystal panel includes a step of arranging an image pick-up device just above the liquid crystal panel; a step of applying illuminating light from the ring illuminating lamp turned on toward the end face of the liquid crystal panel and image-picking up, by the image pick-up device, the light reflected from the side of the liquid crystal panel illuminated with the illuminating light; binary-digitizing, by an image processing device, the image picked up by the pick-up device, detecting the external shape of the liquid crystal panel on the basis of a frame-like white image appeared in the image and detecting the chip or crack of the end face of the liquid crystal panel from the presence/absence from the other white image.
In the defect detecting method disclosed in the above JP-A-2001-153816, the bottom face of the glass sheet is illuminated by the illumination from a lower-longitudinal direction to the rounded end face of the glass sheet so that only the chip appearing at the edge of the glass sheet shines and the other area of the glass sheet becomes completely dark. Thus, of various defects, the shining chip can be detected, but other defects such as the scratch, non-polishing or stain cannot be detected.
Further, in the device disclosed in JP-A-06-258231, illuminating light is applied, from two diagonal upper and diagonal lower directions, to the seaming area diagonally grinding-cut at the ridgeline of the edge of each both the front and back faces of the glass sheet. Therefore, the scratch or stain in the seaming area of the glass sheet can be detected to a certain extent. However, in the process for manufacturing the glass sheet of a flat display panel having the rounded end face, for e.g. the liquid crystal display or plasma display, the glass sheet is heat-treated at a high temperature so that the fine scratch or chip existing at the end face or seaming area of the glass sheet may probably lead to the crack of the glass sheet. Therefore, in the step of manufacturing the glass sheet of the flat display panel, fine defects including chip, non-polishing as well as the scratch and stain must be detected more sophisticatedly than in the device disclosed in JP-A-06-258231.
Further, in the device and method disclosed in JP-A-2003-247953, the ring illuminating lamp having a diameter larger than the external size of the liquid crystal panel is arranged on the outer periphery of the liquid crystal panel and the camera is arranged above the center of the liquid crystal panel. In this arrangement, the scratch, crack and chip on the surface of the liquid crystal panel and the crack and chip on the end face are simultaneously checked. Therefore, the ring illuminating lamp must be changed according to changes in the size of the liquid crystal panel.
In addition, since the liquid crystal panel is generally square and the ring-shaped illuminating lamp is arranged on the outer periphery thereof, the interval between the liquid crystal panel and the ring illumination is not constant. Thus, the intensity of the illumination on the end face of the liquid crystal panel is not constant.
Further, the ring illumination is upsized according to upsizing of the liquid crystal panel so that the facility will be inevitably upsized. Since the camera is provided only above the center of the liquid crystal panel, the angle of image-picking up the end face changes. Thus, as the case may be, the fine defect on the end face such as the scratch, omission of polishing, non-polishing or stain cannot be detected accurately.
Further, where the entire periphery of the end face of the glass sheet is polished in a rounded shape, if the ring illumination is arranged on the one side of the glass sheet and the area from the center to the end face is image-picked up, the end face will shine partially greatly. So, it was difficult to make the intensity of the illumination on the end face uniformly constant. It was also difficult to detect the defects on the end face and at the boundary line between the polished face and glass face by the same optical system.